Korean J Radiol 8(6), December 2007 531

Hydatid Disease Involving Some RareLocations in the Body: a Pictorial Essay

Hydatid disease (HD) is an endemic illness in many countries, and it poses animportant public health problem that’s influenced by peoples’ socioeconomic sta-tus and migration that spreads this disease. Although rare, it may occur in anyorgan or tissue. The most common site is the liver (59 75%), followed in fre-quency by lung (27%), kidney (3%), bone (1 4%) and brain (1 2%). Other sitessuch as the heart, spleen, pancreas and muscles are very rarely affected.Unusual sites for this disease can cause diagnostic problems. This pictorial essayillustrates various radiological findings of HD in the liver, spleen, kidney, pan-creas, peritoneal cavity, omentum, adrenal, ovary, lung, mediastinum andretroperitoneum. Familiarity with the imaging findings of HD may be helpful inmaking an accurate diagnosis and preventing potential complications.

ydatid disease (HD) is a parasitic disease that’s most commonly caused bythe larval stage of Echinococcus granulosus. It is still a considerablehealth problem in the world (1). E. granulosus can reach any organ or

tissue in the body where it develops into small hydatid cysts (1 5). The wall of thecyst contains three layers. The outermost layer is the pericyst, the middle layer is thelaminated membrane and the innermost layer is the germinal epithelium (endocyst)(1 3, 5). The latter is the only living part of the hydatid cyst and it produces thelaminated membrane and infectious scolices that represent the larval stage of theparasite (5). The characteristic imaging findings have been described as calcification ofthe cyst wall, the presence of daughter cysts and membrane detachment (3). However,the radiological signs are usually non-specific. Serologic tests may be helpful in thediagnosis, but their reliability is not 100% (5). Unusual sites of this disease canfrequently cause diagnostic problems and so this can give rise to an increased risk ofdiagnostic delay and many potentially serious complications. Even a mortality rate of4% for this disease was reported in the literature (6, 7). In this paper, we describe theradiologic findings of HD in some uncommon sites besides the liver and lung, as basedon our experiences.

RADIOLOGIC FINDINGS

A pathology-based classification for this disease has been described by Lewall (1).The early lesions (type I hydatid cysts [HCs]) have a non-specific anechoic cysticappearance on ultrasonography (US) (1 4). On CT scans, Type I HC appears as awell-defined, round or oval cystic mass with an attenuation density near that of water(3 30 HU) (Fig. 1). On magnetic resonance imaging (MRI), HC is seen as hypointenseon T1-weighted images and as marked hyperintense lesions on T2-weighted images.

Murvet Yuksel, MD1

Gulen Demirpolat, MD1

Ahmet Sever, MD2

Sevgi Bakaris, MD3

Ertan Bulbuloglu, MD4

Nevra Elmas, MD2

Index terms:Hydatid DiseaseComputed tomography (CT)Magnetic resonance (MR)Ultrasound (US)

Korean J Radiol 2007;8:531-540Received November 23, 2006; accepted after revision April 18, 2007.

1KSU Medical School Department ofRadiology 46050 Kahramanmaras-Turkey; 2Ege University Medical SchoolDepartment of Radiology 35100 Izmir-Turkey; 3KSU Medical School Departmentof Pathology 46050 Kahramanmaras-Turkey; 4KSU Medical School Departmentof General Surgery 46050Kahramanmaras-Turkey

Address reprint requests to:Murvet Yuksel, MD, KahramanmarasSutcu Imam Universitesi, Tip FakultesiRadiology Anabilim Dali 46050Kahramanmaras -Turkey.Tel. +90 344 - 221 23 37 Fax. +90 344 - 221 23 71e-mail: myuksel@ksu.edu.tr

H

MRI can displays a low-signal-intensity rim that surroundsthe cyst (the “rim sign”), which is more evident on the T2-weighted sequences (2, 8).

For type II HCs, many daughter cysts and/or matrixdevelop within the parent cyst with or without cyst wallcalcification (Fig. 2) (1). Daughter cysts, indicating viabil-ity, have a lower attenuation value than that of mothercysts on CT scans (Fig. 2). On MRI, daughter cysts mayappear slightly hypointense or isointense relative to thematernal matrix on the T1-weighted images and hyperin-tense on the T2-weighted images. When present, floatingmembranes are seen as low signal intensity linearstructures within the cyst on both the T1- and T2-weightedimages (2, 3, 8).

Type III HCs represent a calcified, non-viable degener-ated cyst (Fig. 3) (1). US demonstrates cyst calcification ashyperechoic areas with a strong posterior shadow. Cystcalcification can be seen as round, hyperattenuating areason CT and as hypointense areas on MRI (2, 3).

Complications of HC include rupture and superinfectionof type I and II cysts (1). Rupture occurs in 50% 90% ofcases. Three types of rupture can occur: contained,

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Fig. 1. Type I hydatid cyst of the liver in an 11-year-old girl. Un-enhanced CT scan of the upper abdomen shows a large unilocu-lar hydatid cyst (18 HU) with a high-attenuation wall in thesubdiaphragmatic portion of the liver (arrow).

Fig. 2. Type II hydatid cyst in a 36-year-old man. Contrast-enhanced axial CT scan of the upper abdomen demonstratescystic lesion with peripheral daughter cysts and wall calcificationin the left lobe of the liver. Note the daughter cysts have a lowerattenuation value than the mother cyst (arrows).

Fig. 3. Type III hydatid cyst in a 28-year-old man. Denselycalcified lesion is seen on the right upper quadrant of theabdomen on barium radiography of the stomach.

Fig. 4. Direct rupture of hydatid cyst into the peritoneal cavity in a50-year-old man. Contrast-enhanced axial CT scan of the upperabdomen demonstrates a partially collapsed cystic lesion (blackarrow) with an irregular contour growing exophytically from theliver. It has lost its normal spherical shape. Isoattenuatingdetached membranes (white arrow) appear as serpentinestructures within the lesion. A small amount of ascites (whitearrowhead) is present.

communicating and direct (1). In contained rupture, theendocyst ruptures and becomes detached from thepericyst. In communicating rupture, the cyst ruptures intoan anatomical diversion structure like the biliary orbronchial tree (1). Direct rupture occurs when both theendocyst and the pericyst are ruptured with the cysticcontents spilling into the pleural and peritoneal cavities ora hollow viscus (Fig. 4) (1). The cyst may become consider-ably smaller and less spherical both in communicating anddirect ruptures (1). MRI may demonstrate disruption in the

low-signal intensity rim of the cyst wall and extrusion ofthe contents through the defect (3, 8).

LIVER

The liver is the most common site of HC. The cysts maycause pain, discomfort, abdominal swelling or a palpablemass or thrill (5, 6). In the early stages of the disease, theappearance of HCs may be uncharacteristic and mimic thatof simple cysts. However, the double-line sign can often beseen on sonography in unruptured HCs (Fig. 5) (3, 4).

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Fig. 5. Type I hydatid cyst of the liver in a 42-year-old woman. AtUS examination of liver, the cyst wall is seen as doubleechogenic lines separated by a hypoechoic layer (the double-linesign) (small white arrows). This finding helps to differentiatehydatid cysts from simple cysts, cystic tumors, pseudocysts ormetastases.

Fig. 6. Type II hydatid cyst is seen on US examination of the liverin a 37-year-old man. Multiple echogenic sand (small whitearrows), which is composed of brood capsules and scolices, fallsto the dependent portion of the cyst by repositioning the patient.This finding is referred to as the snowstorm sign. Note the earlyseparation of the laminated membrane from the germinalmembrane (large white arrow).

Fig. 7. A. Large type II hydatid cyst of the liver in a 12-year-old girl. Contrast-enhanced transverse CT scan of the upper abdomendemonstrates cystic lesion with a peripheral daughter cyst (short arrow). There is also evidence of pericystic biliary ducts dilatationsecondary to the mass effect by a large echinococcal cyst (long arrow).B. Microscopic section of hydatid cyst in the same patient shows a laminated membrane that is lined inside by the germinal layer(arrowheads). There is also a scolex (arrow) close to the germinal layer. (Hematoxylin & Eosin staining; original magnification 10)

A B

Simple liver cysts do not demonstrate internal structures(2, 3), although multiple echogenic foci due to hydatidsand may be seen within the lesion by repositioning thepatient on sonography (Fig. 6) (5). On MRI, a low-signal-intensity rim can be helpful to differentiate a unilocular HCfrom a simple liver cyst (2). Multiple unilocular cysts areindistinguishable from polycystic disease (2, 5). Whenechinococcal cysts become enlarged, they can producepericystic biliary tract dilatation due to the mass effect (Fig.7) (2, 5, 9). Decreasing intracystic pressure, endocystdegeneration, host response, trauma and medical treatmentcan cause separation of the endocyst from the pericyst (2,3). Complete collapse of the endocyst results in asonographic water-lily sign when the parasite lies in themost dependent part of the cyst or this produces an irregu-lar, solid echo pattern (Fig. 8) (5). The wall of HC, even

without calcification, is typically seen as a high-attenuatedstructure on unenhanced CT (Fig. 1) (3). The calcificationmay occur in the cyst wall or internally in the cyst, and thisis detected on radiography in 20% to 30% of liver HCs(Fig. 3) (2, 3, 5). Following the formation of HC, manypotential local complications may develop such as rupture,infection (Fig. 9), perforation to the biliary tree (up to 90%of HCs) (Fig. 10), and involvement of the portal venoussystem (Fig. 11), diaphragm and thoracic cavity (0.6%16% of the cases with hepatic HCs) (2, 3).

LUNG

The lung is the second most common location ofhematogenous HC spread in adults and it is probably themost common site in children (2, 5). Pulmonary hydatid

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Fig. 8. Type II hydatid cyst mimicking a solid mass in a 25-year-old man. US image demonstrates a mixed echogenic lesion(arrows) of the liver. Note the serpentine structures within thematrix, which represents the collapsed membranes.

Fig. 9. Infected hydatid cyst of the liver in a 25-year-old woman.Contrast-enhanced CT scan shows a cystic lesion, including airbubbles and collapsed membranes, in the right lobe of the liver(black arrow).

Fig. 10. Rupture of hydatid cyst into the biliary tree in a 54-year-old woman. A, B. The axial MR image of the upper abdomen using a fast spin-echo T2-weighted sequence shows a hyperintense cystic lesion (shortwhite arrow), including hypointense linear structures representing detached membranes, in the right lobe of liver. There is thehypointense rim (black arrowhead) at the periphery of the lesion. MRI reveals cyst wall defect (black arrow) and the passage of detachedmembranes through a defect into the extrahepatic bile duct (white arrowhead). Note the biliary dilatation (long white arrows) due toobstruction of the extrahepatic bile duct by hydatid membranes.

A B

cysts are often asymptomatic, and they are usually foundas incidental findings on routine chest radiography, butoccasionally symptoms occur due to the pressure effects onadjacent structures (5). The most prominent radiologicalfinding is a dense, round, well-demarcated opacity that canresemble a neoplasm. Calcification (0.7% of cases) anddaughter cysts are rarely seen in lung HD (2, 10). Whenthe growth of the cyst produces erosion in the bronchioles,air between the endocyst and pericyst can produce a“crescent or inverse crescent sign” between the cyst walland the pericyst (Fig. 12). If air continues to enter the cyst

cavity, then the “water lily” sign can be seen (an endocystmembrane floating in the most dependent part of thepericyst cavity) (2, 11). The radiologic appearances ofinfected cysts are similar to those of a lung abscess: a thick-walled cavity with an air-fluid level and surroundingpneumonia (Fig. 13) (5). When HCs are infected, they cancause a solid appearance and give rise to a diagnostic errorsuch as malignant tumor. Other complications of lung HCinclude rupture and recurrent acute pulmonary embolism(2).

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Fig. 11. Portal vein involvement of hydatid disease in a 20-year-old woman. Theaxial (A) contrast enhanced spin-echo T1-weighted sequence (fat-suppressiontechnique) shows a low-signal-intensity lesion in the subdiaphragmatic portion ofthe right hepatic lobe (white arrow). Thrombosis is identified within the branches ofthe portal vein (small black arrows). On MR angiography (B), portosystemic venouscollaterals (white arrowhead) are seen and the portal vein is not visualized.

A

B

Fig. 12. Multiple lung hydatid cysts in a 30-year-old woman. Axialcontrast-enhanced CT of the thorax shows multiple type I hydatidcysts in the lung. There are also ruptured hydatid cysts in thelower lobe of the right lung with the inverse crescent sign (arrow)due to air between the endocyst and pericyst.

Fig. 13. Infected pulmonary hydatid cyst in a 5-year-old boy. Axialcontrast-enhanced CT scan with a mediastinal window settingshows multiloculated low-attenuation lesion with a thick wall in theright lung.

KIDNEY

Renal HD is rare (3% of case), usually solitary andlocated in the cortex (2). These patients may present with aflank mass, dysuria, pyuria, hematuria, persistent fever,renal stones, hypertension or renal colic (10). Hydatiduriacan occur after rupture of the cyst into the collectingsystem (5). Any form of HC can be seen in renal HD (Fig.14) (2, 3, 10). Mural calcification and daughter cysts oftencoexistent. These findings are helpful in the differentialdiagnosis of HCs from a simple renal cyst, necrotic renalcell carcinoma, renal abscess and infected cysts, but itsometimes can be difficult to differentiate HC fromnecrotic renal cell carcinoma since calcifications may beencountered in both lesions (2, 10).

PERITONEAL CAVITY

Peritoneal HC, either primary or secondary, representsan uncommon but significant manifestation of this disease(approximately 13%). It is always secondary to traumaticor surgical rupture of a hepatic, splenic or mesenteric cyst(2, 3, 10). CT is the modality of choice for these patientsbecause it permits imaging of the entire abdomen andpelvis (3). The lesions are generally multiple and any typeof HC can arise anywhere in the peritoneal cavity (Fig.15). Unilocular cysts (type I) should be distinguished frommesenteric cysts or intestinal duplication cysts (2, 6, 10).

SPLEEN

Primary splenic involvement is very rare (less than 2%)(Fig. 16). The symptoms are mainly abdominal pain,splenomegaly and fever. Splenic HCs are usually solitary,and their imaging characteristics are similar to those ofhepatic HCs (2, 10). Other splenic cystic lesions such asepidermoid cyst, pseudocyst, splenic abscess, hematomaand cystic neoplasm of the spleen should be considered inthe differential diagnosis (10).

PANCREAS

Primary HC of the pancreas is rare, representing 0.2

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Fig. 14. A 44-year-old-man with type II hydatid cyst of the leftkidney. After albendazole therapy, detached germinalmembranes are seen within the cyst as low signal intensity linearstructures on coronal T2-weighted spin-echo MRI (white arrow).

Fig. 16. Large type II hydatid cyst with primary splenic involve-ment in a 50-year-old woman. Contrast-enhanced axial CT scanof the abdomen demonstrates unenhanced, well-definedhypodense lesion with a hydatid matrix and peripheral daughtercysts (arrows) in the upper part of the spleen.

Fig. 15. Intraperitoneal hydatid cyst in a 40-year-old woman witha history of liver involvement and previous surgery. The contrast-enhanced axial CT scan revealed multiple cystic lesions (arrows)and ascites (asterisk) in the pelvis.

2% of all human infestation. It is usually single and locatedin the head of the pancreas (2, 10). The clinical symptomsdepend on the size and location of the cyst within thepancreas. The lesion in the head of pancreas frequentlypresents with jaundice due to obstruction of the commonbile duct. However, the body or tail lesions rarely causesymptoms. The differential diagnosis includes abscess orcystic neoplasms of the pancreas (10). The typical appear-ance and location of HCs within the pancreas is notestablished due to their relatively rare occurrence (2). Inthis paper we present the MRI features of primary HC ofthe pancreas located in the tail (Fig. 17). We also presentthe CT findings of a partially calcified HC located in thetail of the pancreas (Fig. 18).

ADRENAL GLAND

The adrenal gland is an extremely rare location for HCand it is usually involved as a part of systemic echinococco-sis. It is usually asymptomatic and these patients usually

present with the symptoms that are caused by space-occupying lesions (12). The imaging features depend on thestage of evolution of the disease (Fig. 19). Unilocular HCsshould be differentiated from exophytic renal cysts thatoriginate from the upper pole of the kidney (2).

OVARY

Ovarian involvement is also very rare and generallysecondary to peritoneal spread of daughter cysts due torupture of a liver HC. Ovarian HCs are usually asympto-matic and they can be discovered incidentally or they maycause irritation or compression symptoms. The ovarianlesion may be unilocular (Fig. 20) or contain daughter cyststhat can give rise to a multiloculated appearance, andovarian lesion should be considered in the differentialdiagnosis of cystic pelvic masses such as cystadenoma orcystadenocarcinoma (2).

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Korean J Radiol 8(6), December 2007 537

A B

Fig. 17. Type II hydatid cyst with primary pancreas involvement ina 57-year-old woman.A. Axial MR image of the upper abdomen shows low-signal-intensity lesion with peripheral daughter cysts on the T1-weightedsequence (fat-suppression technique) (arrow). Daughter cysts areseen as lower-signal-intensity lesions compared to the mother cyst. B. On axial MR image of the upper abdomen, daughter cysts areseen within the mother cyst (short black arrow). The mother cystand daughter cysts are seen as hyperintense lesions on the fastspin-echo T2-weighted sequence. Also note the hypointense rim atthe periphery of the lesion (long white arrow).C. Microscopic section of hydatid cyst following distal pancreatec-tomy demonstrates a thick laminated membrane (star) with a thingerminal layer (long arrow). Short arrow shows the pancreasparenchyma.

C

RETROPERITONEUM

Isolated retroperitoneal HCs are also uncommon andthey are usually secondary to the involvement of otherorgans or to previous surgery. Any type of HC can be seenin the retroperitoneum (2, 13). In this paper we present theCT findings of a case of retroperitoneal type II HC thatoriginated from the right psoas muscle (Fig. 21). There wasno evidence of any other organ involvement in this case.

OMENTUM

Isolated omental HC is one of the most unreported sites(14). Information about the appearances of HCs within theomentum is inadequate due to their relatively rare

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Fig. 18. Partially calcified hydatid cyst of the pancreas in a 27-year-old woman. Contrast material-enhanced CT scan shows ahypodense lesion with dense peripheral calcification (arrow) inthe tail of the pancreas.

Fig. 19. Type II hydatid cyst of the right adrenal gland in a 47-year-old woman. The axial (A) spin echo T1-weighted MR imagedemonstrates low-signal-intensity hydatid cyst with peripheral daughter cysts (arrows) in the right adrenal gland. Note that the daughtercysts have lower signal intensity compared to the mother cyst. The sagittal (B) plane spin echo T2-weighted MR image shows linearstructures within the mother cyst, which represent detached membranes (long black arrow), and round, nodular lesions within the mothercyst, which represent daughter cysts (short black arrows).

A B

Fig. 20. Incidentally found type I hydatid cyst of the ovary in a 39-year-old woman. The axial (A, B) contrast-enhanced CT scan shows aunilocular low-attenuation lesion (arrows) in the right ovary.

A B

occurrence. In our reported case, a unilocular HC waslocated in the lesser omentum (Fig. 22).

MEDIASTINUM

Mediastinal HCs are also rare and they can be solitary ormultiple lesions. The symptoms and complications of cystdepend on the size, location and involvement of adjacentstructures. The imaging appearance can vary from type I totype III (2) (Fig. 23). The HCs in the mediastinum shouldbe differentiated from cysts of a bronchogenic, pleuroperi-cardial, thymic or enteric origin, and from intramuralesophageal cysts such as lymphangioma and anteriormeningocele (3).

CONCLUSION

The occurrence of E. granulosus in some locations of thebody is very rare. These anatomic locations may causedifficulties in making the differential diagnosis as E.granulosus is usually not suspected in some locations of thebody. Imaging modalities such as US, CT and MRI arehelpful in diagnosing this disease. Radiologists, surgeonsand physicians should always consider HD in differentialdiagnosis of a cystic lesion, and especially for the cysticleasions encountered in patients who live in or have comefrom endemic regions and if any of the previouslydescribed imaging features (e.g., calcification, daughtercysts and/or intracystic membranes) are seen. Familiaritywith the various imaging appearances of HD may preventdiagnostic delay, and so decrease the risk of life-threaten-ing complications.

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Fig. 21. Type II retroperitoneal hydatid cyst in a 25-year-oldwoman. The axial contrast-enhanced CT scan through the pelvisdemonstrates a hydatid cyst with daughter cysts (arrow) adjacentto the right psoas muscle.

Fig. 22. An omental hydatid disease in a 36-year-old man. Theaxial contrast-enhanced CT scan obtained through the stomachshows unilocular low-attenuation lesion (arrow) in the lesseromentum, which represents type I hydatid cyst.

Fig. 23. A mediastinal hydatid disease in a 20-year-old man. Theaxial contrast-enhanced CT scan shows a well-defined, low-attenuation lesion (arrow) that is consistent with type I unilocularhydatid cyst. Compression of the superior vena cava by thehydatid cyst is also seen.

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